How to make your BB shooting replica more accurate

I like BB shooting replicas.  The use of BBs makes it possible to replicate the function of semi-auto firearms more accurately than is possible in replicas that shoot pellets.  However, I find it frustrating that so few BB replicas shoot well.  One of the first replicas I owned was a Tanfoglio Witness, and I loved its heavy weight and the way it felt just like a cartridge firing 1911.  But I found it irritating that, even at six yards, it scattered BBs over a 5” circle.  Given that current replicas are generally well made, surely they can be made to shoot with a little more accuracy than that?

The main problem here is consistency.  Pellet shooting replicas are good at sending pellets on a very similar trajectory every shot, leading to satisfactorily small groups.  BB shooters are affected by tiny imperfections in BBs and the barrel which leads variation in the trajectory of the BBs and larger groups.  But there are things you can do to make your BB shooting replica produce smaller groups.

How does it all work?

The first thing to consider is what happens when a pellet or BB travels down the barrel of an air or airsoft gun.  A .177” pellet (or a .177” lead ball) fits tightly into the barrel of an air gun and is squeezed against the rifling on the inner surface of the barrel.  When you fire the gun, gas pressure builds up behind the pellet until this is sufficient to overcome the friction holding the pellet against the sides of the barrel.  When the pellet starts to move forward, the rifling also causes it to spin.  When the pellet leaves the end of the barrel, it continues to spin, improving stability.  The friction caused by the pellet being squeezed against the sides of the barrel is the reason that pellets always leave the barrel with less speed than BBs.  The accuracy (or otherwise) of your pellet shooting airgun is largely dependent on how accurately the barrel was made in the first place and how much the rifling has eroded over time.  A build-up of deposits on the rifling can cause some minor degree of inconsistency in the flight of the pellet (though some lead build-up can actually improve accuracy), but generally the most important factor is how straight the barrel is in the first place.

Shooting a pellet in a rifled barrel

Now let’s look at what happens when you shoot a BB through a smoothbore barrel (I’ll talk about shooting BBs in rifled barrels in a moment).  And it doesn’t matter whether we’re talking about a 4.5mm steel BB or a 6 or 8mm plastic BB, the mechanics are the same.  The BB does not fit tightly inside the barrel.  A typical 6mm airsoft BB for example, is actually around 5.95mm external diameter while the barrel on most modern airsoft guns is anywhere from 6.04 – 6.08mm internal diameter.  So when the gas is pushing the BB down the barrel, some leaks past the BB and forms a thin layer of gas between the BB and the inside of the barrel.  Because of this, the BB doesn’t actually touch the sides of the barrel at all and this thin layer of gas actually helps to stabilize the BB and keep it travelling straight.

Shooting a BB in a smoothbore barrel

There are couple of things to think about here.  First, barrel length.  It takes time for the BB to stabilize on the layer of gas.  When it first enters the barrel, the BB tends to bounce off the inner sides of the barrel, especially if it hits a hop-up rubber on the way.  After it has travelled some distance, this bouncing is dampened down and the BB stabilizes in the centre of the barrel.  There is some argument about how long a barrel must be in order for the BB to stabilize fully, but most people seem to agree that anything less than around 70mm (a little under three inches) is unlikely to allow the BB to stabilize completely.  In general terms, the longer the barrel, the better stabilized the BB will be when it leaves the muzzle.

The second thing to consider is hop-up.  Most airsoft guns and some steel BB shooting guns have hop-up.  This is a rubber nub inside the barrel and close to the breech.  The nub is located on the top of the barrel and projects inside.  As a BB travels down the barrel, it strikes the rubber nub which causes it to spin backwards.  This backspin helps to overcome the force of gravity and allows the BB to maintain a flatter trajectory after it leaves the muzzle.  On many guns, the amount which the nub projects into the barrel (and therefore the amount of backspin) can be adjusted.  Most people will tell you that the effects of hop-up are not evident at ranges below 10m, but I haven’t found this to be entirely true.  Even when shooting at 6m, I have found that adjusting hop-up can affect the vertical point of impact of BBs by an inch or so.  However, hop-up initially de-stabilises the path of the BB through the barrel.  So, on a gun with hop-up, it may take more distance for the BB to stabilize.

Hop-up nub inside the barrel of 4.5mm ASG CZ75

OK, now let’s talk briefly about shooting steel 4.5mm BBs through a rifled barrel.  Some pellet shooting guns can also fire steel BBs.  Many manufacturers and some suppliers talk about .177” and 4.5mm as if they’re the same calibre.  They are not – a 4.5mm steel BB is notably smaller than a .177” pellet.  If you shoot a steel BB through a .177” rifled barrel, it is not large enough to engage with the rifling.  Instead, just as in a smoothbore barrel, it floats on a layer of gas in the centre of the barrel.  However, the flow of this layer of gas is much less stable than on a smoothbore barrel because of the rifling which causes it to swirl and tumble.  Also, as it initially enters the barrel and bounces off the sides, the hard steel BB can cause erosion and damage to rifling over time.  A BB will always leave the barrel travelling faster than a pellet because of the lack of friction, but in my experience, I have not come across any replica air pistol which shoots BBs accurately though a rifled barrel.  The higher speed at which BBs travel is unimportant and because of the lack of accuracy and the possibility of damaging rifling, I’d suggest that you shoot steel BBs only in guns which have smoothbore barrels and only shoot pellets or .177” lead balls in those which have rifled barrels.

Left, shooting eight .177” pellets from a replica with a rifled barrel (in this case, an Umarex H&K P30) at 25 feet, aim point is the centre of the black circle. Right, same replica, same range, same aim point but this time using eight steel 4.5mm BBs. As you can see, the steel BBs give notably less accuracy.

How to improve things

Right, so, now we know how it all works, how can we make our BB shooting guns more accurate?  If we’re talking about airsoft guns, the first thing many people think about is a tightbore barrel.  As the name suggests, these are aftermarket barrels which have a smaller internal diameter than the original.  That sounds good in theory, but I’m not totally convinced.  The critical thing that determines how straight your BB will travel is how well the BB stabilizes inside the barrel. Part of what determines this is the size of the layer of gas between the outside of the BB and the inside of the barrel.  Too big a layer is bad and can cause the BB to be unstable.  But, too small a gap is also bad and can prevent the BB from stabilizing fully.  If you do fit a tightbore barrel, you can expect to see your replica shooting with more power – less gas is lost round the BB and so more is available to propel it down the barrel.  However, I suspect that most accuracy gains which users report after fitting these parts come as much from improved tolerances in the manufacturing process used when making these aftermarket barrels compared to the processes used in creating the original barrel as from the tightness of their bore.  An expensive aftermarket barrel may be straighter than a more cheaply made original part (though there is no guarantee of this) but if the bore is too tight, it can actually make consistency worse.

If you don’t want to buy new bits, what else can you do?  Well, there are two things that affect the way the BB travels down the barrel. The first is the quality of the BB itself.  The layer of gas between the BB and the barrel is very thin – around 0.05mm.  That’s equivalent to about the width of two human hairs.  So, any tiny imperfection in the BB which is spinning after hitting the hop-up rubber can cause instability in the flow of gas and may cause the BB to move erratically in the barrel.  The closer to being perfectly spherical that your BBs are, the more consistently they will shoot.  If you can see seams or other moulding marks on your BBs, they are obviously not going to perform well. 

This is a pair of cheap and very nasty Chinese 6mm plastic BBs with clearly visible seams and moulding marks.  Very few 6mm BBs are this obviously crap, but no matter how good your replica, it’s never going to shoot consistently with poor quality BBs.

However, even if they look glossy and smooth, not all BBs are equal.  In general, you should avoid brightly coloured or transparent BBs (especially those which have visible bubbles of air inside them), any small packs of BBs which are supplied with an airsoft gun and any BBs which are not identified by weight.  Most BBs which are made in Japan are good as are the majority from Taiwan.  In my experience, Chinese BBs can be of comparatively poor quality and should generally be avoided.  Just because it says “Precision” or “High Quality” on the packaging is no guarantee that BBs are good.  Be prepared to try different brands and pay a little more for quality plastic BBs and when you can, choose those from manufacturers you recognize (Guarder and KWA, for example, produce very high quality 6mm BBs).

Hopefully, it’s also obvious that re-using plastic BBs isn’t a good idea. The plastic used to manufacture 6 and 8 mm BBS is fairly soft, so they tend to develop flat spots when they hit a target. This of course makes them unstable if you re-use them. While we’re talking about plastic BBs, it’s also worth thinking about weight. In general, the heavier the BB, the more stable it will be and so the smaller groupings you’ll see. You may have to experiment with different weights of BB to find one that works best for you, but the table below gives a general guide to the most appropriate weight BBs to use in your replica. The fps figures are based on the speed when shooting with standard 0.2g BBs.

Under 300 fps: 0.12g

300 – 350 fps: 0.2g

350-400fps: 0.25g

400-450fps: 0.28g

450 – 500fps: 0.36g

Over 500fps: 0.43g

I haven’t found the same variation in quality with 4.5mm BBs.  Most steel BBs from the big producers seem to be of very high and consistent quality with few blemishes or imperfections whether they say Blaster, Umarex, Crosman or ASG on the pack.  I tend to avoid steel BBs from unknown manufacturers – there are Chinese steel BBs around and though I haven’t tried them, I’d probably like to keep it that way.  I don’t use copper coated BBs because I find that they leave deposits on the inside of the barrel, though I know that many other shooters use them without problems.  I also don’t use lead balls in guns with smoothbore barrels intended for steel BBs either.  These lead balls are slightly larger than the 4.5mm steel BBs and they are often not perfectly spherical (or even if they start out that way, the soft lead can deform as they move through the feed system).  They also tend to leave deposits on the inside of the barrel. Lead balls are fine in rifled barrels, but generally not good in guns with smoothbore barrels.

The second thing that affects the way in which a BB travels down the barrel is the cleanliness of that barrel.  On a smoothbore barrel, any tiny speck of dust or other contamination on the inside surface can cause disruption to gas flow which will affect BB stability.  I have found that cleaning the inner barrel is the best way to quickly improve groupings and to reduce the number of flyers on any BB shooting gun.  Even a new replica will likely have traces of packing grease inside the barrel.

Cleaning the inner barrel is very simple.  Remove the barrel if possible, or at least dismantle the replica so that you can easily access both ends of the inner barrel.  If your replica has adjustable hop-up, turn it completely off (i.e. so that the rubber nub protrudes as little as possible into the barrel).  Make a simple pull-through using a piece of cord or string and a piece of clean, absorbent cloth. Do be careful what you use for a pull-through – many inner barrels are made of very light alloy and it’s frighteningly easy to cut the end of the barrel if you use a hard cord or wire pull-through. Soak the cloth in warm water which has a little washing-up liquid in it and pull through several times.  Finish off by doing the same again with a clean, dry piece of cloth.  That’s it!  Re-adjust the hop-up, re-fit the barrel and you will now have an inner barrel which is free of particles or deposits which are likely to affect the stability of the BB.

Cleaning the barrel from an ASG CZ75

Use top quality BBs and try shooting your replica after cleaning the barrel (and after re-adjusting the hop-up if fitted) and I think you’ll notice a marked improvement.  Groups should be noticeably smaller. Gas flow is critical on any BB shooter and gas flow through the barrel and around the BB is the single place where you can generate the most marked improvement.  Go on, give it a try.  And let me know if it works for you.

Related Posts

Are all steel BBs the same?

Umarex Beretta PX4 Storm Redux

Regular readers (hello to you both!) may have noticed that I have already posted a review of the Umarex PX4 Strom back in 2014. That was based on my ownership of two PX4s between 2010 and 2013. So, you might be wondering what more I have to say about this particular replica? Well, here’s the thing; I rather like the PX4 but for me, it was marred by one particular issue – namely an inability to shoot to the point of aim. Both my PX4s shot sufficiently high and to the left at 6m that I didn’t bother with them much.

Which was a pity because there’s actually a lot to like about this air pistol. It’s a licensed blowback replica with full markings and, being a pellet shooter with a rifled barrel, it should be an accurate shooter. I have read on other sites that, somewhere around 2014/2015 the design of the PX4 was changed and that it’s now better. I have also read elsewhere that the accuracy issues on this replica are caused by the plastic outer-barrel shroud interfering with the flight of the pellet as it leaves the end of the barrel, and that over time this decreases as the plastic is worn away.

I intend to re-visit the PX4 to find out whether either of these things are true and, if they are not, to look at how I can otherwise improve how it shoots. So this review will be a little different as it will include comparison between the PX4 I have now and my earlier experience as well as information on how it can be improved as a shooter. Will I end up with an accurate shooter or yet another PX4 that gathers dust in the back of the gun cupboard? Let’s find out…  

Real Steel Background

Beretta launched the PX4 Storm in 2004 as a replacement for the 8000 (Cougar) series of pistols, a more compact alternative to the Beretta 92. The PX4 is a polymer framed semi-automatic pistol that uses the rotating barrel locking system from the 8000 series and the trigger and safety system from the Beretta 92.

This was Beretta’s second polymer-framed handgun and it borrows many of the design cues from the first, the Beretta 9000 which was designed by the Giorgietto Giugiaro Design group. The PX4 is still in production and has proved popular as a law enforcement weapon in the US, Canada, Italy and Argentina amongst many others.

Unlike most other Beretta pistols, the PX4 has a fully enclosed slide. It is available as several models both with and without a manual safety and chambered for 9mm, .40 S&W and .45 ACP. Its distinctive looks have led to the PX4 featuring in a number of television shows and several movies Inception (2010), The A-Team (2010), Taken 2 (2012), Robocop (2014), and John Wick Chapter 2 (2017).

Leonardo DiCaprio with PX4 in Inception (2010)

The Umarex Beretta PX4

The Umarex Beretta PX4 was launched in 2008 and was at that time only the second pellet-shooting blowback replica from that company – the first, the Magnum Research Desert Eagle appeared in 2005. This a CO2-powered blowback replica with a polymer frame and a metal slide. It has a four-inch rifled barrel and can shoot both .177 pellets and 4.5mm steel BBs stored in a double-ended magazine that hold up to eight BBs or pellets in each end. This is a fully licensed replica that features Beretta markings.

It is manufactured in Japan on behalf of Umarex, though I don’t know who actually makes it. I have seen a number of claims that this replica was updated and (perhaps?) improved around 2014. I have not been able to confirm this and most claims are a little vague about precisely what changes have been made to the original model. Part of the reason for this review is to find out whether this really is a new model and whether it’s any better than the original.   

Specification:

Calibre: 4.5mm BBs/.177″ pellets

Magazine capacity: Sixteen .177″ pellets or 4.5mm steel BBs

Propellant: CO2

Barrel length: 4″, rifled

Weight: 720g (1.59 pounds)

Overall length: 7½”

Sights: Fixed front and rear with white dots

Action: SA/DA

Packaging and presentation (3/5)

My Umarex PX4 came in a sturdy card box that contains the pistol, a single magazine and a multi-language user manual. The box did seem a little better than most, being strong and provided with things like a small square of foam to stop the manual safety from rubbing against the box lid.

This is not the same box that either of my previous PX4s came in. So, at the very least, I can say confidently that the box has changed…

Visual accuracy 7/10

Externally, this PX4 seems identical to both my earlier models. It’s a pretty fair representation of the original including overall size and shape and full Beretta markings.

The only visual anomalies are the manual safety on the right of the frame, the base of the grip and safety markings on the right of the slide and frame, though at least these aren’t highlighted in white. The slide-mounted manual safety and slide release catch are moulded in place and have no function, but at least they look pretty good and don’t detract from the overall appearance. 

Functional Accuracy 6/15

Functionally, this is identical to my earlier PX4s. The trigger, hammer and magazine work as per the original but the slide doesn’t move through a full range, it doesn’t lock back on empty and the slide release doesn’t work. You can lock the slide back, simply by pulling the trigger while the slide is held to the rear, but this really serves no purpose. The manual safety does feature a decocker – when you move it to the “S” position, it safely drops the hammer, and that’s quite useful if you are leaving this replica with CO2 in it (though you shouldn’t do that for extended periods or it can degrade the CO2 seal). To move the manual safety from “S” to “F,” you have to pull back on the serrated centre section, a fingernail-breaking job. Happily, to move it in the opposite direction, you don’t need to do this.  

Shooting 38/45

OK, so for me, this is the most important part of this review – does this PX4 shoot any better than the previous examples I owned? The first thing I looked at was the plastic outer barrel shroud – I have seen claims that this can interfere with the flight of the pellet or BB as it leaves the inner rifled barrel. It’s certainly notable that this plastic outer barrel is very close to the inner barrel and, if you look closely, it isn’t perfectly circular – there are irregularities that look like mould marks that project inwards. If you look at the image below, you’ll see one at around the two o’clock position.

Is it possible that these deflect the pellet as it leaves the rifled barrel? We’ll have to wait and see. Getting the Umarex PX4 ready to shoot is straightforward. The lower rear part of the grip is removed to reveal the CO2 chamber.

The bottom of the grip is twisted clockwise, the CO2 cartridge is inserted and then the knurled wheel is finger tightened. Then, the base of the grip is turned counter-clockwise and this pierces the CO2. I get full four magazines to a single CO2 – that’s over 60 shots, which isn’t too bad for a blowback replica though the last couple of shots do sound as if they’re a little down on power.

Back in 2008 when this replica was first released, the magazine was pretty novel, though similar designs have since been used in a number of other replicas. It’s double-ended and at each end there are eight chambers into each of which you can load a pellet or BB.

Only two chambers are visible at a time and you then have to click the magazine round to expose the next two. It’s a little fiddly and time-consuming but fairly easy. Pellets are a friction fit and steel BBs are retained by a magnet. With this replica loaded it’s time to shoot, and the first thing I want to talk about is the trigger. It works in both double and single action and the double action pull is predictably long and heavy. However, even in single action it’s a long pull and the first 70% or so does nothing – there is barely any resistance until you come to the point where the next pellet is indexed in the rotary magazine. This isn’t especially heavy, but it happens just before the release point. My technique with this pistol is to pull the trigger past the indexing point and to pause just before the release before the final pull. But,  more than once, I found myself unintentionally shooting as I pulled the trigger past the indexing point and through the release point. This isn’t a nasty trigger, it’s just different and, if you use it as I do, it’s worth being fairly careful.

Finally, it’s time to shoot. And it soon becomes apparent that this new PX4 isn’t as bad in terms of accuracy as my previous versions – it’s much worse! The image below shows the result of sixteen shots at 6m range using Umarex 0.48g Mosquito flat-fronted pellets. The aim point was the base of the centre black circle.

This was taken after more than 150 shots, to give the barrel a chance to lead-in. As you can see, the shots are scattered mainly to the right of the target and not all sixteen have even hit my 14cm square target. I tried a couple of other types of target pellet and all gave similar results. This is very disappointing – shooting pellets through a rifled barrel should give much tighter groups than this. Most of my airsoft and BB shooting replicas can do better than this! Both my previous PX4s were capable of 1 – 1½” groups at 6m, though they were high and to left of the aim point. But, this is also puzzling. You can actually see the pellets deflecting to the right as they leave the barrel. What’s going on here and, most importantly, can it be fixed?

Intermission – Improving the PX4

OK, so it’s time to find out what the problem is. I have read in other sites a suggestion that inaccuracy is caused by the front face of the outer plastic barrel interfering with the flight of the pellet. And, if you look closely, you can see that the inner and outer barrels aren’t quite concentric and that the lower part of the plastic barrel does seem to overlap with the bore of the inner metal barrel.

To test whether this is causing the problem, I want to enlarge the hole in the outer barrel. To avoid the possibility of damaging the rifled barrel, I’ll disassemble first and fortunately, this isn’t difficult (you’ll find a link to a disassembly guide at the end of this review). To remove the slide and remove the firing valve only requires the removal of three pins and once the valve is out, you can remove the inner barrel and the plastic breech block.

Then, I widened the opening in the outer barrel and this is the result. Yes, I know, it’s pretty rough – I’ll clean it up later but all I want to do for the moment is see if this makes a difference to shooting. You actually have to be pretty careful when you’re doing this. The end of the plastic barrel is the only thing that retains the inner barrel in place – if you remove too much material, you could make this replica inoperable.

I put it all back together, load up with the same pellets, and here’s the result of sixteen shots at 6m.

That’s quite a dramatic improvement. The vertical spread was previously over six inches. Now, it’s down to around 1½”. So, the the pellets were hitting the plastic outer barrel and that was making a difference to accuracy. Now, I want to look at whether I can move the centre of the group to the left to coincide more closely with the point of aim and to tighten it up further.

I disassemble again and this time, I’m looking at the fit of the rifled barrel into the plastic breech block. It’s notable that the fit is quite loose – the metal barrel pushes into the breech block and I note that it can move around quite a lot and the fit in the plastic block is all that gives it support.

I go for the simplest solution – a small strip of adhesive tape round the base of the barrel makes it a tight push-fit into the breech block and it no longer has any play. I reassemble and try shooting again. Here’s the result, with the aim point still at the base of the inner black circle.

That’s more like it! The group is still a little to the right, but it’s now close to spot-on for elevation and the overall group is just over 1”.

Shooting – Part 2

Now that I have the PX4 shooting reasonably, I enjoy it much more. This is actually a very nice replica to shoot. It has good weight, the blowback is strong and the pellets hit the target with a great deal of power (my chrony shows around 350 – 360fps). And now, by aiming off a little to the left, I can get reasonable groups close to the centre of the target. Here’s the result of a full magazine, fired fairly rapidly from 6m.

These few simple mods to the PX4 have transformed it as a shooter. Out of the box, it was so inaccurate that shooting was basically a waste of time. Now, I can appreciate the good things about this replica and actually enjoy shooting it. I like the fact that it isn’t too loud, I appreciate the fact that I can get 60 shots per CO2 and I especially like that this replicates the feel of shooting the real steel more closely than many replicas. The score for this section is for this pistol after I had done these few mods. As it was out of the box, it would have scored much lower.   

Quality and reliability 11/15

One of the things I didn’t like about both my pre-2014 PX4s was that the slide didn’t fit well and rattled annoyingly from side to side. That problem has vanished now – on this version the slide fits very well indeed and the movement of the slide feels more precise and better engineered. Overall, this feels like a good quality replica and I haven’t experienced any functional problems with loading CO2, pellets or with shooting.

But…

The problem with the plastic outer barrel fouling the pellets as they leave the barrel is pretty poor and any sort of reasonable quality control should have picked this up long ago. It isn’t difficult to fix, but really, you shouldn’t have to think about fixing a new replica. Likewise the loose fit of the rifled metal barrel in the plastic breech block – this means that you are never going to get tight groups and it just shouldn’t have left the factory like this. Like the problem with the outer barrel, it’s easily fixed, but it should need this done just to get it to shoot straight.

So, I’m conflicted as to what to say here about quality. This does seem like a good quality replica in many ways, but I got was the basis for something to work on rather than a pistol that shoots well out of the box. Now, it’s possible that I was unlucky and that I got a poorly finished and assembled PX4, but looking at other reviews online, I’m not the only one to experience a lack of accuracy with this replica. If you are lucky enough to get a good one, this is a great replica. If you get one like mine, you’ll have to be prepared to do a little work to turn it into an acceptable target shooter.   

Overall Impression 8/10

This looks and feels like a well made and finished replica. It’s solid, the action of the slide now feels very good and the trigger, though it takes some getting used to, actually isn’t too bad. I guess the best thing I can say about it is that, now that the problems are fixed, I really enjoy shooting it.

Conclusion

Overall, this does feel a little different to the two previous versions of this replica that I owned before 2014. The fit of the slide and its action are now much better and overall it looks and feels good. However, as a shooter, it was simply dire out of the box. With a little work I have managed to change that and I now do really enjoy shooting the PX4.

Whether you choose to buy one of these probably depends on whether you feel confident to disassemble and make some small changes. You might be lucky and get one that doesn’t need these things done, but if you get one like mine, you will probably not be prepared to put up with its atrocious accuracy out of the box. So, if you’re willing to put in a little time and effort, this can be made into a very decent replica for not a great deal of cash.

Total Score (after modification) – 73/100

Links

Here’s a link to a step-by-step disassembly guide for the PX4. Remember, to do the mods described here, you only need to remove the slide and the firing valve – that means you only have to remove three pins.

Related Posts

Original Umarex Beretta PX4 review

WE Bulldog (Beretta PX4 Storm) review

Umarex Desert Eagle review

Umarex Buck Mark URX trigger job

If you have read my review of the Umarex Buck Mark URX, you’ll know that I was quite impressed with it – for a budget airgun, it isn’t a bad shooter. However, for me at least, the shooting experience was not as good as it might be because of a heavy trigger-pull. The point of this article is to find out if this can be easily be improved. Caution is required here – as far as I am aware, it isn’t possible to buy spares for this replica, so if you mess things up, there is probably nothing you can do other than buy another. So, it’s sensible to go step-by-step, doing only a little at a time.

Disclaimer. Airguns can be dangerous. Taking them to bits and especially messing with things like trigger assemblies can make them shoot differently, unpredictably or even (if you are particularly unlucky) not at all. So, don’t do this modification unless you are confident about your air-gunsmithing skills. In fact, don’t even think about it… And if you do go ahead anyway and mess it up, don’t come complaining to me!

It’s also worth mentioning that you probably shouldn’t test fire this replica without loading a pellet. On most spring-powered air pistols, the effort required to force a pellet down the rifled barrel creates a cushion of air inside the piston chamber. If you fire without a pellet in the barrel, the piston slams forward with much greater force and this can damage the seal.

OK, with that stuff out of the way, let’s look at disassembly. Which fortunately is pretty straightforward and requires only a decent cross-head screwdriver, a 3mm hex key and something to drift out small pins. Before you start, make sure the pistol is not cocked!

Here is our starting point. Six shots, six metres, freestanding, Umarex Mosquito 4.3gr pellets. This is fairly typical of the groups that I’m seeing with this pistol before modification. I have shot smaller groups, but it always feels like I’m fighting the trigger pull. It isn’t terrible, but I am certain this replica is capable of better.

Let’s start disassembly. First, remove the two cross-head screws, arrowed, to release the grips.

Take them off and you’ll be left with this plastic frame.

Next, remove the two 3mm hex-headed screws that retain the accessory rail (the longer rear one also joins the upper and lower body).

Finally, remove the two cross-head screws, one either side just ahead of the trigger guard (arrowed).

And that’s it. Now gently lever the upper body from the frame.

Now we can see the trigger mechanism and the sear (arrowed).

When the gun is cocked, the sear engages with a slot in the front of the piston (arrowed).

When you pull the trigger, the sear moves down and allows the piston to move forward, propelled by the main spring. All that is making the trigger heavy is friction between the sear and the piston.

One thing that becomes immediately obvious with the gun disassembled is just how heavy the trigger spring is – my improvised gauge suggests that it alone accounts for almost three pounds of the pull weight. That’s a fair chunk of the total, so perhaps cutting a coil off the spring may make things better?

Before I do that, it’s time to look at the sear itself and think about what I can do to make it better? I’m going to do this step-by-step to try to find what, if anything, makes a notable difference.

The obvious answer is to decrease friction by simply lubricating the sear and the front of the piston housing. I did this using a PTFE grease with Teflon, which is also safe for use on replicas because it doesn’t degrade seals.

I reassembled the gun and tried shooting, and the result was; no discernible difference in the trigger pull. I wasn’t entirely surprised – on disassembling the pistol, it has obviously been well lubricated in the factory.  

For the next steps, it’s easier if you remove the sear completely. This is fairly simple. Drift out the two pins (arrowed) from the right – both pins have knurled ends, and must be removed and reinstalled from the left.

Then, the complete trigger and safety mechanism can be lifted out of the top of the frame.

This also gives access to the trigger spring, which is below the end of the sear and mounts on to a plastic pin moulded into the frame.

Next, remove the cross-head screw, arrowed, and remove the safety. This gives access to the single pin that retains the sear – again it must be drifted from the right and removed to the left.

With the sear removed, polishing is much easier. The face you need to polish is arrowed.

The next option is to try hand-polishing the sear. I don’t want to remove much material here, only to make the sear slide more easily as the trigger is pulled and hand-polishing removes very little metal. I polished the sear using Autosol-sovol metal polish until it looked nice and shiny.

Then I reassembled, and the result was, no difference. Hmm…

Next, I cut a single coil off the base of the trigger spring. You have to be careful here – the trigger spring keeps the sear pressed against the piston when the gun is cocked. Remove too much tension from the spring and the gun may not cock properly. Below you can see before (above) and after shots of the trigger spring. Removing only one coil does make the trigger-pull lighter and it does not affect function. You could remove more coils to make it lighter still, but there will come a point where the spring won’t have enough force to lock the sear into the piston.

With everything reassembled, the uncocked trigger feels noticeably lighter but, when I try shooting, there is no obvious difference and certainly very little improvement.

OK, time for the final and most drastic step – using a Dremel attachment in a power drill to polish the sear more aggressively. This is where you can all too easily turn a functioning replica into a wall-hanging, so you really must be patient, do a little at a time and then reassemble and test.

I do some initial polishing, reassemble, and I can’t really feel much difference. So, I try again and remove a little more. You can see the result below – I have actually removed very little material and only on the upper part of the face of the sear.

This time, when I reassemble, the trigger does feel different. Before, the trigger felt like an on/off switch – you pull, there is no response and you continue to pull harder until, suddenly, it releases. Now, you pull and you can actually feel the sear moving until it reaches the release point. That’s better, and it does feel lighter. And, just as it did out of the box, it continues to lighten-up more as I shoot.

Six shots, six metres, freestanding, Umarex Mosquito 4.3gr pellets after the final round of grinding. It took around twenty shots or so before the trigger lightened up enough to make this sort of group fairly easy to achieve.   

Finally, there is an appreciable difference! I could go on to remove more material in an effort to make the trigger lighter still, but there is a risk that I’ll get to the point where the gun may not cock properly or may even fire when the safety is released. At the moment, it’s better than it was and everything works as it should, so I’m going to stop there.  

Was it worth the effort? I think so. This is not a difficult replica to disassemble and reassemble and the difference in trigger-pull is noticeable and this directly translates to tighter groups. If you are fairly confident in your ability to work on airguns and patient enough to go step-by-step, I recommend this as a distinct improvement over the original.

Happy shooting!

Related posts

Umarex Buck Mark URX review